Vibrating wire piezometers provide a number of advantages over the traditional hydraulic piezometer design. There are currently many methods and configurations for installing vibrating-wire piezometers, the most common being: single piezometers in sand packs (SP), multilevel piezometers in sand packs (MLSP), and fully-grouted multilevel piezometers using either bentonite (FGB) or cement-bentonite grout (FGCB). This study assesses the performance of these four different installation methods at a field site possessing complex stratigraphy, including glacial and marine sediments. To accomplish this objective, pore pressure data recorded between December 2017 and July 2019 were analyzed. Data indicate that SP, MLSP, and FGB piezometers performed most reliably, based on the fact that piezometers installed at the same depth with these methods recorded similar pressure variations that were coherent with the hydrogeological setting. Of the two fully-grouted installations using cement-bentonite grout, one installation failed completely due to a hydraulic short circuit, likely caused by preferential flow occurring along the wires of the embedded instruments. The lack of a standard method for mixing cement-bentonite grout at the time of construction likely contributed to the failure of the FGCB installations, as the grout mixture used in this study was likely too viscous to provide a suitable seal.

Field performance of four vibrating-wire piezometer installation methods

Bias in hydraulic head measurements from multilevel vibrating-wire piezometers with excessively permeable backfill

Landslides in sensitive post-glacial marine clays are one of the major geological hazards in Canada, Norway and Sweden. Current hydrogeological conceptual models used for slope stability analyses in these deposits consider simple groundwater flow conditions within a homogenous, isotropic, massive clay deposits, where fractures are surficial features that only exist within a 1–5 m-thick weathered zone. This study uses cross-correlation analysis on hydraulic head data from a large network of vibrating-wire piezometers in clay deposits along the St. Lawrence River and in the Saguenay-Lac St-Jean Lowlands, in Quebec, Canada, to show that hydraulically-active fractures are present to depths of up to 16 m at 4 (possibly 6) of the 7 locations studied. These findings suggest that current conceptual models have a high likelihood of misrepresenting local flow systems, and that further field and modeling work is needed to characterize the extent and influence of these fracture networks. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2023GL103156

Deep Hydraulically‐Active Fractures in Sensitive Clay Deposits: Implications for Groundwater Flow and Slope Stability

Landslides in sensitive post‐glacial marine clays are one of the major geological hazards in Canada, Norway and Sweden. Current hydrogeological conceptual models used for slope stability analyses in these deposits consider simple groundwater flow conditions within a homogenous, isotropic, massive clay deposits, where fractures are surficial features that only exist within a 1–5 m‐thick weathered zone. This study uses cross‐correlation analysis on hydraulic head data from a large network of vibrating‐wire piezometers in clay deposits along the St. Lawrence River and in the Saguenay‐Lac St‐Jean Lowlands, in Quebec, Canada, to show that hydraulically‐active fractures are present to depths of up to 16 m at 4 (possibly 6) of the 7 locations studied. These findings suggest that current conceptual models have a high likelihood of misrepresenting local flow systems, and that further field and modeling work is needed to characterize the extent and influence of these fracture networks.

The thick sequences of marine clayey deposits that blanket the St. Lawrence Lowlands in south-eastern Canada are highly susceptible to landslides. With 89% of the population of the province of Queb...

/pdf/hydrogeology-of-a-complex-champlain-sea-deposit-quebec-du4qo859xk.pdf

Alexandra Germain

Papers

Hydrogeology of a complex Champlain Sea deposit (Quebec, Canada): Implications for slope stability

Field performance of four vibrating-wire piezometer installation methods